1. Field of the Invention
This invention relates to a misfire detecting device for an internal combustion engine which detects a misfire which occurs in the engine for instance when the ignition system is out of order.
2. Prior Art
For instance when, in an internal combustion engine, the ignition system becomes out of order, a large quantity of gas is discharged, so that the catalyst generates an abnormally high temperature. In order to prevent this difficulty, a device has been proposed in the art which detects the variation of an operating parameter, such as the angular speed of the crank shaft of the internal combustion engine or the cylinder internal pressure, representing the state of combustion, thereby to inform the operator of the abnormal condition by means of an alarm lamp or the like. Conventional misfire detecting devices of this type have been disclosed by Japanese Patent Application (OPI) No's 19532/1983 and 26345/1987 (the term "OPI" as used herein means an "unexamined published application").
The device disclosed by the aforementioned Japanese Patent Application (OPI) No. 19532/1983 operates as follows: When the difference between the crank shaft's angular speeds before and after the stroke of expansion in the internal combustion engine is smaller than a predetermined value, it is determined that a misfire has occurred. On the other hand, the device disclosed by the Japanese Patent Application (OPI) No. 26345/1987 operates as follows: The cylinder internal pressure of the engine is detected with a cylinder internal pressure sensor, and a crank angle is obtained with which the cylinder internal pressure is maximum. When the crank angle is within a predetermined crank angle range, it is determined that the combustion is normal.
That is, it is determined from the angular speed of the crank shaft or the cylinder internal pressure every combustion cycle whether the combustion is normal or the misfire occurs, and when a misfire occurrence rate (the frequency of occurrence of misfires per 100 combustion cycles or 100 revolutions of the engine) is larger than a predetermined value, a misfire detection signal is produced to turn on an alarm lamp or the like.
The conventional misfire detecting device is designed as described above. Hence, in the device, in order to prevent the generation of the abnormally high temperature by the catalyst due to misfires, or in order to make protection even in the operation highest both in speed and in load which is most greatly affected by misfires, a low threshold value is provided for determination of the misfire occurrence rate; that is. the sensitivity of detection is decreased.
Therefore, even in a low speed and low load operation in which the rotation is liable to be changed by factors other than the misfire for instance as in the case where an internal combustion engine operated vehicle is shocked or vibrated during traveling, and in which no actual damage occurs even when misfires occur to the extent that the misfire occurrence rate exceeds the criterion, the misfire detection signal is erroneously produced. That is, the misfire detecting operation is low in reliability.
Furthermore, the misfire detecting device disclosed by the aforementioned Japanese Patent Application (OPI) No. 19532/1983 is so designed as to detect misfires in all the operating conditions. When the load of the internal combustion engine is considerably light as in the case where it is run at high speed with no load, the friction loss of the internal combustion engine is small, or the inertial energy is large, and therefore the variation in angular speed of the crank shaft due to misfires is smaller than in the case where the load is heavy. As a result, it is impossible for the device to accurately detect misfires, or, even if the detection sensitivity is increased, misfires are erroneously detected because of external noises. That is, the result of detection provided by the misfire detecting device is low in reliability. Hence, for instance in the case where the device is coupled to a display unit, to indicate a failure in the ignition system according to the result of detection, to inform the operator of it thereby to request for repair, the following difficulties may be involved: Although the combustion is abnormal, it is not indicated; or although the combustion is normal, it is indicated as abnormal. That is, the operator cannot rely on the indication based on the result of detection provided by the device.